1. Field of the Invention
The invention relates to methods of drilling a circuit board, and in particular to drilling a printed circuit board with a laser operated at an energy density below the ablation threshold of a conductive layer on the circuit board.
2. Background Information
Microvia formation has become a key factor in high density printed circuit board (PCB) and printed wiring board (PWB) is fabrication, especially for increasingly sophistic electronics such as light, portable system and high density integrate circuits. Most advanced PCBs and PWBs are multi-layered and require a lot of metallized holes to interconnect conductors in different layers. Mechanical drilling techniques have many disadvantages and Laser drilling has become popular because of its superior features. Several different types of lasers are used for hole drilling including CO2, YAG/UV, and excimer lasers. CO2 lasers operate at a wavelength of 9.3-10.6 micrometers. Typical resins used in the dielectric layer of circuit boards have a high absorption threshold at this wavelength. However, the copper conductive layer has a very low absorption threshold (circa 1%) at this wavelength. Therefore, CO2 lasers are suitable for drilling holes in the resin substrate, but the top copper layer will reflect most (circa 99%) of the laser. Currently, a two step process is used. Firstly, a hole is produced in the copper conductor layer with photolithography and etching methods similar to those used in semiconductor manufacture, and then the hole in the resin is drilled using a CO2 laser.
On the other hand, a frequency-tripled YAG laser operating is at 355 nm can be used for drilling the hole in both the copper conductor and dielectric layers. However, this requires a spiral drilling process, which limits production efficiency.
Excimer lasers are also useful for drilling both the copper conductor and dielectric layers. However, they are not widely used in PCB manufacture because of their expense.
It is an object of the present invention to provide a method of laser drilling a circuit board in which the laser is operated at an energy density below the ablation threshold of a conductive layer of the board.
According to a first aspect of the invention there is provided a method of drilling a circuit board comprising a dielectric material provided with a conductive layer having an ablation threshold, and in which the laser is operated at an energy density below the ablation threshold, the method including:
providing on the conductive layer a ablation layer having an ablation threshold near or below that laser energy density,
directing the laser at a target area of the ablation layer to ablate portions of the conductive and dielectric layers in the target area, and
removing any remaining ablation layer from the conductive layer
Preferably, the laser is a CO2 laser operated at a wavelength of 9 to 11 micrometers.
Preferably, the ablation layer is metal.
Preferably, the ablation layer is Tin.
Preferably, the ablation layer is 3 micrometers thick.
Preferably, the ablation layer is provided on the conductive layer by electroplating.
According to a second aspect of the invention there is provided a method of drilling a circuit board using a CO2 laser operating at a wavelength of between 9 to 11 micrometers, in which the circuit board comprises a dielectric material provided with a copper layer, the method including:
providing a metal layer on the copper layer,
directing the laser at a target area of the metal layer to ablate portions of the copper and dielectric layers in the target area, and
removing any remaining metal layer from the copper layer.
Preferably, the metal layer is Tin.
Preferably, the metal layer is 3 micrometers thick.
Preferably, the metal layer is provided on the conductive layer by electroplating.
Further aspects of the invention will become apparent from the following description, which is given by way of example only.